Explanation Two topics are discussed, related to the identification of LAM cells in LAM nodules and to the response of LAM patients to sirolimus and simvastatin. First, with regard to the identification of LAM cells in LAM lung nodules as well as within other involved organs: Lymphangioleiomyomatosis (LAM) is characterized by the proliferation in the lung, axial lymphatics (e.g. lymphangioleiomyomas) and kidney (e.g., angiomyolipomas) of abnormal smooth muscle-like LAM cells, which express melanoma antigens such Pmel17/gp100 and have dysfunctional tumor suppressor tuberous sclerosis complex (TSC) genes TSC2 or TSC1. Histopathological diagnosis of LAM in lung specimens is based on identification of the Pmel17 protein with the monoclonal antibody HMB-45. We compared the sensitivity of HMB-45 to anti-peptide antibody aPEP13h, which reacts with a carboxy terminal peptide of Pmel17. LAM lung nodules were laser-capture microdissected to identify proteins by Western blotting. HMB-45 recognized 25% LAM cells within the LAM lung nodules, whereas aPEP13h identified over 82% of LAM cells within these structures in 90% of patients. Whereas HMB-45 reacted with epithelioid but not with spindle-shaped LAM cells, aPEP13h identified both spindle-shaped and epithelioid LAM cells, providing greater sensitivity for detection of all types of LAM cells. HMB-45 recognized Pmel17 in premelanosomal organelles; both antibodies recognized a Pmel17 variant of 50 kDa. Based on its sensitivity and specificity, aPEP13h may supplant HMB-45 for use in the diagnosis of LAM and related disorders expressing melanogenic proteins. Second, with regard to treatment of LAM patients with sirolimus and simvastatin: Combined simvastatin and sirolimus therapy reduced TSC2-null lesions and alveolar destruction in a mouse model of LAM, suggesting that combined sirolimus plus simvastatin therapy may benefit LAM patients. Our objective was to determine whether simvastatin changed the prevalence of adverse events or altered the therapeutic effects of sirolimus. The study was based on a retrospective review of patient data in our LAM cohort. Adverse events for 14 simvastatin plus sirolimus, 44 sirolimus, and 20 simvastatin patients and changes in lung function before and after treatment were recorded. Sirolimus-related adverse events in the simvastatin plus sirolimus and sirolimus only groups, were 67 and 66 % for stomatitis, 60 and 61 % for hypercholesterolemia, 47 and 52 % for diarrhea, 47 and 45 % for peripheral edema, 40 and 61 % for acne, 40 and 30 % for hypertension, 27 and 27 % for proteinuria, and 33 and 27 % for leukopenia. The frequency of simvastatin-related adverse events in the simvastatin and sirolimus plus simvastatin groups were 50 and 60 % for arthralgias, and 36 and 35 % for myopathy. Before simvastatin plus sirolimus therapy, FEV1 and DLCO rates of change were respectively, -1.40.2 and -1.80.2 % predicted. After simvastatin plus sirolimus therapy, these rates changed to +1.20.5 (p=0.635) and +0.30.4 % predicted, respectively (p=0.412). In 44 patients treated with sirolimus alone, FEV1 and DLCO rates of change were -1.70.1 and -2.20.1 % predicted before treatment, and +1.70.3 and +0.70.3 % predicted after therapy (p<0.001). Based on these data, therapy with simvastatin plus sirolimus does not increase the drug adverse events or alter the therapeutic effects of sirolimus.